This invention relates generally to computer disk drives and related control systems for controlling the position of one or more actuator heads with respect to one or more associated memory storage disks or the like, for purposes of accurately reading and/or writing data within closely spaced tracks on the disk surfaces. More particularly, this invention relates to an improved and simplified control system for automatic compensation of head position offset errors of the type generally attributable to dc offsets of the disk drive control circuitry.
In recent years, microcomputer equipment particularly such as so-called personal and desk top computers have become extremely popular for a wide variety of business and educational and recreational and other uses. Such computers commonly include a main central processor unit having one or more memory storage disks for storage of data. In many modern computers, the storage disk or disks are provided as part of a Winchester-type disk drive unit, sometimes referred to a "hard" disk, wherein the storage disks are supported on a rotatable spindle within a substantially sealed housing. The disks are rotatably driven in unison by a small spindle motor, and one or more electromagnetic heads are displaced by a head actuator assembly to traverse surfaces of the rotating disks for purposes of reading and writing data. Such Winchester-type disk drive units are generally preferred in comparison with so-called floppy disk drives due to their higher memory storage capacities and faster operating speeds.
For a typical Winchester-type disk drive, data is recorded onto magnetizable surfaces or surface films on the memory storage disks in the form of data bits adapted to be encoded and decoded in binary form. The data is recorded with the confines of narrow concentric tracks defined on the disk surfaces, with track densities in the range of about 500 to 1,000 and more tracks per inch being relatively well known. Accordingly, for any selected disk drive unit, the total memory storage capacity is directly proportional to the number of available data tracks.
In normal operation of the disk drive unit, a system controller is provided for identifying the location of data which has been stored by recording upon the storage disks, typically through the use of directory name, file name, and/or track address information. When reading of the data at a specific data track is desired, the system controller signals the head actuator assembly to displace the heads to the desired position. Alternately, when data recording or writing is desired, the system controller operates through the actuator assembly to move the heads in a manner aligning one head with a vacant data track.
For optimum data recording and subsequent retrieval, it is necessary for the heads to be positioned in substantially centered and accurate tracking alignment with respect to any selected one of the closely spaced data tracks. In this regard, the disk drive unit normally includes a servo system having means for detecting servo data bits which are prerecorded onto one or more of the storage disks. The servo system responds to detection of the servo data to adjust the actual head position for obtaining optimum centered head alignment with a selected track.
While available servo systems function generally satisfactorily for most operating conditions, such servo systems have not provided a simple and economic way to compensate for so-called position offset errors attributable to dc offsets of the disk drive circuitry. That is, the actual head position as controlled by the actuator assembly is subject to a variety of electronic and mechanical error factors, such as component characteristics variations, track length, cable bias, temperature, bearing friction, etc., all of which can result in head position offset errors of a magnitude which varies across the radial span of the disk or disks. Servo system detection and response to prerecorded servo data does not adequately compensate for such position offset errors during all operating conditions of the disk drive.
There exists, therefore, a significant need for a relatively simple, cost-efficient, and reliable system and method for offsetting or nulling dc position offset errors in a computer disk drive or the like. The present invention fulfills these needs and provides further related advantages.